Plasma-assisted high-power microwave generator

Abstract

A high-power microwave/mm-wave oscillator is filled with an ionizable gas at a pressure of about 1-20 mTorr, into which an electron beam is injected at a high current density of at least about 1 amp/cm 2 , but typically 50-100 A/cm 2 . A plasma is formed which inhibits space-charge blowup of the beam, thereby eliminating the prior requirement of a magnet system to control the beam. The system functions as a slow-wave tube to produce narrow-band microwaves for a gas pressure of about 1-5 mTorr, and as a plasma wave tube to produce broadband microwave/mm-wave radiation for a gas pressure of about 10-20 mTorr. A new high output, hollow-cathode-plasma electron gun is employed in which a metal oxide layer is formed on the inner surface to enhance the secondary electron yield; a cathode, grid, and extraction anode have respective sets of multiple apertures which are mutually aligned to yield a high perveance beam; the cathode, grid, and anode are curved to geometrically focus the beam, and a beam with a circular cross-section is generated.

We claim: 
     
       1. An oscillator for generating electromagnetic radiation within the microwave to millimeter-wave range, comprising: a waveguide housing,   means for introducing an ionizable gas into said waveguide housing,   an electron gun for injecting an electron beam into said waveguide housing, and   means for maintaining the gas pressure within said waveguide housing at a level sufficiently low to avoid a voltage breakdown of the beam, and sufficiently high to provide enough ions to substantially neutralize space-charge expansion of the beam,   said electron gun injecting said beam into the waveguide housing with a sufficient current density to at least partially ionize the gas therein and generate electromagnetic radiation at said gas pressure.   
     
     
       2. The oscillator of claim 1, wherein said gas pressure is maintained within the approximate range of 1-20 mTorr. 
     
     
       3. The oscillator of claim 2, implemented as a slow-wave tube, said waveguide housing having a rippled wall, wherein said gas pressure is maintained within the approximate range of 1-5 mTorr. 
     
     
       4. The oscillator of claim 2, implemented as a plasma wave tube, wherein said gas pressure is maintained within the approximate range of 10-20 mTorr. 
     
     
       5. The oscillator of claim 1, wherein said electron gun generates a beam with a current density of at least about 1 amp/cm 2 . 
     
     
       6. The oscillator of claim 5, said electron gun comprising a hollow cathode having an outlet, an apertured grid at said cathode outlet, means for introducing an ionizable gas into said hollow cathode, means for establishing an electrical glow discharge between the cathode and the grid to generate a plasma within said cathode, the grid having a generally high transparence but with apertures small enough to prevent the passage of plasma through it, a generally transparent anode on the opposite side of the grid from the cathode, and means for applying an electrical potential to said anode to extract an electron beam from the plasma behind said grid. 
     
     
       7. The oscillator of claim 6, wherein the inner cathode surface is formed from a chemically active metal, and said gas introducing means includes means for doping the gas with a trace amount of oxygen to react with said metal and form an oxide thereof, thereby enhancing the secondary electron yield from the cathode. 
     
     
       8. The oscillator of claim 6, wherein said hollow cathode and anode have respective sets of apertures which are mutually aligned to yield a high perveance beam. 
     
     
       9. The oscillator of claim 6, wherein the cathode surface, grid and anode are curved concave with respect to the beam to geometrically focus the beam. 
     
     
       10. The oscillator of claim 6, said hollow cathode being cylindrical to generate an electron beam with a substantially circular cross-section. 
     
     
       11. The oscillator of claim 1, wherein said means for introducing an ionizable gas into the waveguide housing also introduces said ionizable gas into the electron gun at a pressure approximately equal to the pressure within the waveguide housing. 
     
     
       12. The oscillator of claim 11, said electron gun including means for establishing an electrical glow discharge through the ionizable gas within said gun to establish a plasma therein, said plasma providing an electron source for said beam. 
     
     
       13. The oscillator of claim 12, said electron gun including means for producing said discharge in pulses of about 1-100 μsecond duration. 
     
     
       14. The oscillator of claim 1, said electron gun injecting an electron beam into one end of the waveguide housing, and further comprising a horn antenna at the opposite end of the waveguide housing for emitting output electromagnetic radiation. 
     
     
       15. An oscillator for generating electromagnetic radiation within the microwave to millimeter-wave range, comprising: (a) a waveguide housing,   (b) an electron gun coupled to said waveguide housing for injecting an electron beam into said waveguide housing,   (c) means for introducing an ionizable gas into the waveguide housing and electron gun at a pressure sufficiently low to avoid a voltage breakdown of the beam, and sufficiently high to provide enough ions within the waveguide housing to substantially neutralize space-charge expansion of the beam, and   (d) said electron gun comprising: (i) a hollow cathode having multiple outlets to said waveguide housing,   (ii) a perforated grid located adjacent to said multiple cathode outlets, said grid having apertures small enough to prevent the passage of plasma,   (iii) means for establishing an electrical glow discharge between the cathode and the grid to generate a plasma within the cathode,   (iv) a perforated anode on the opposite side of the grid from t he cathode, and   (v) means for applying an electrical potential to said anode to extract an electron beam from the plasma behind the grid into said waveguide housing,     said electron gun generating said beam with a sufficient current density to at least partially ionize the gas therein and generate electromagnetic radiation.   
     
     
       16. The oscillator of claim 15, wherein said cathode has an inner cathode surface formed from a non-magnetic metal. 
     
     
       17. The oscillator of claim 15, wherein said cathode has an inner cathode surface formed from a chemically active metal, and said gas introducing means includes means for doping the gas with a trace amount of oxygen to react with said metal and form an oxide thereof, thereby enhancing the secondary electron yield from the cathode. 
     
     
       18. The oscillator of claim 15, wherein said cathode outlets and anode have respective sets of apertures which are mutually aligned to yield a high perveance beam. 
     
     
       19. The oscillator of claim 15, wherein said cathode, grid and anode are curved concave with respect to the beam to geometrically focus the beam. 
     
     
       20. The oscillator of claim 15, said hollow cathode being cylindrical to generate an electron beam with a substantially circular cross-section. 
     
     
       21. Apparatus for generating a generally non-spreading electron beam, comprising: an electron gun for generating an electron beam,   a housing coupled to the electron gun for receiving the electron beam, and   means for introducing an ionizable gas into said housing for ionization by the beam, said gas being introduced at a pressure at which sufficient ions are generated in the vicinity of the beam to substantially neutralize space-charge blowup of the beam, said gun generating said beam with a sufficient current density to generate electromagnetic radiation within the housing at said gas pressure.   
     
     
       22. The beam generating apparatus of claim 21, wherein said gas is introduced into the housing at a pressure within the approximate range of 1-20 mTorr. 
     
     
       23. The beam generating apparatus of claim 21, wherein electron gun generates said beam with a current density of at least about 1 amp/cm 2 . 
     
     
       24. An improved slow-wave tube, comprising: a ripple-walled waveguide housing,   means for introducing an ionizable gas into said housing at a pressure within the approximate range of 1-5 mTorr, and   an electron gun for injecting an electron beam into said housing with a current density of at least about 1 amp/cm 2 , said electron gun comprising: (a) a hollow cathode having multiple outlets,   (b) means for introducing an ionizable gas into the cathode,   (c) a perforated grid located adjacent to said multiple cathode outlets, said grid having apertures small enough to prevent the passage of plasma,   (d) means for establishing an electrical glow discharge between the cathode and the grid to generate a plasma within the cathode,   (e) a perforated anode on the opposite side of the grid from the cathode, and   (f) means for applying an electrical potential to said anode to extract an electron beam from the plasma behind said grid.     
     
     
       25. The improved slow-wave tube of claim 24, wherein said gas is helium. 
     
     
       26. An improved plasma wave tube, comprising: A waveguide housing,   means for introducing an ionizable gas into said waveguide housing at a pressure within the approximate range of 10-20 mTorr, and   an electron gun for injecting an electron beam into said housing with a current density of at least about 10 amp/cm 2 , said electron gun comprising: (a) a hollow cathode having multiple outlets,   (b) means for introducing an ionizable gas into the cathode,   (c) a perforated grid located adjacent to said multiple cathode outlets, said grid having apertures small enough to prevent the passage of plasma,     (d) means for establishing an electrical glow discharge between the cathode and the grid to generate a plasma within the cathode, (e) a perforated anode on the opposite side of the grid from the cathode, and   (f) means for applying an electrical potential to said anode to extract an electron beam from the plasma behind said grid.     
     
     
       27. The improved plasma wave tube of claim 25, wherein said gas is helium. 
     
     
       28. The improved plasma wave tube of claim 26 wherein said waveguide housing has a smooth cylindrical wall and a single electron beam is injected into said housing to produce a pair of counterstreaming plasma waves. 
     
     
       29. An improved high current electron gun, comprising: a hollow cathode having multiple outlets,   means for introducing an ionizable gas into the cathode,   a perforated grid located adjacent to said multiple cathode outlets, said grid having apertures small enough to prevent the passage of plasma,   means for establishing an electrical glow discharge between the cathode and the grid to generate a plasma within the cathode,   a perforated anode on the opposite side of the grid from the cathode, and   means for applying an electrical potential to said anode to extract an electron beam from the plasma behind said grid.   
     
     
       30. The electron gun of claim 29, wherein the inner cathode surface is formed from a non-magnetic metal. 
     
     
       31. The electron gun of claim 29, wherein the inner cathode surface is formed from a chemically active metal, and said gas introducing means includes means for doping the gas with a trace amount of oxygen to react with said metal and form an oxide thereof, thereby enhancing the secondary electron yield from the cathode. 
     
     
       32. The electron gun of claim 29, wherein said cathode outlets and anode have respective sets of apertures which are mutually aligned to yield a high perveance beam. 
     
     
       33. The electron gun of claim 29, wherein said cathode, grid and anode are curved concave with respect to the beam to geometrically focus the beam. 
     
     
       34. The electron gun of claim 29, wherein said hollow cathode is cylindrical for generating an electron beam with a substantially circular cross-section.